1. Field of the Invention
The present invention relates generally to spacecraft antenna pointing error correction, and specifically to a system and a method for improving spacecraft antenna pointing accuracy utilizing feedforward estimation.
2. Description of the Related Art
A significant trend in satellite communications is the use of spot beams to provide targeted services to specific urban regions and population centers. Examples of A2100 spacecraft that include spot beam payloads are Echostar 7, Rainbow-1, and the Echostar X spacecraft. Given the typical small coverage region diameter of 200 to 400 km, accurate pointing is critical to minimize the necessary beam diameter and payload power. To achieve high accuracy beam pointing, prior art systems use autotrack antenna feeds and receivers that sense the antenna pointing error with respect to an uplink beacon signal. With an autotrack system, antenna circular pointing errors of 0.05 degrees are possible, which is a factor of three better than the typical 0.15 degree pointing error without autotrack. The drawback of prior-art autotrack systems is that they use feedback control strategies that react only to the presently sensed pointing error. When the error exceeds a given threshold, the antenna gimbal is stepped to reduce the error.
Typically, the threshold is set to a value of one gimbal step, so the tracking error will be at least this much and generally more due to latencies in the system implementation. For example, for a step size of 0.012 degrees, the maximum pointing error is roughly 0.02 degrees using a prior-art control approach. This error is excessive, since it represents 40% of the total allowable pointing error of 0.05 degrees due to all sources. Lowering the threshold can reduce the error, but also may cause excessive stepping due to noise that can exceed the gimbal mechanism life capability over the 15 year mission.